The use of flexible printed circuit boards (FPCB) in fiber optic modules is known in the art. An FPCB is typically composed of a single layer of flexible insulating material, such as polyimide, with electrical traces typically made out of copper on at least one side of the insulating layer. The flexible layer serves both as an insulator and as a carrying medium for electrical traces. A FPCB may include more than one such insulating layer with electrical traces appearing between and at outer surfaces of any one of those layers. In order to route electrical signals to and from the embedded layers, the FPCB may contain electrically conductive passthrough holes, also known in the art as vias. The advantage of the FPCB over the rigid printed circuit board is that FPCB can be bent to fit into various compact electronic products. FIG. 1 illustrates a conventional fiber-optic module utilizing a FPCB with a single bend. The electrical traces on the FPCB 100 are used to bring electrical signals to and from the opto-electronic and electronic chips 102 and 104 to electrical interconnects 108 on the bottom of the fiber-optic module. The opto-electronic chips 102 include lasers or detectors, or both, while the electronic chips 104 include laser driver circuits or receiver circuits, or both. The electrical interconnect 108 couples the fiber-optic module to a large host printed circuit board (not shown). Alternatively, the interconnects 108 to the host printed circuit board may appear on the back 114 of the module. The host board will typically hold any number of fiber optic modules. A rigid piece of material 106 can be mounted to the FPCB 100 near the opto-electronic 102 and electronic chips 104, and another rigid piece of material 110 can be mounted to the FPCB 10 near the interconnects 108 to provide structural support. More electronic or opto-electronic processing circuitry 112 can also be provided on the rigid material 110.
There are three functional groups of fiber-optic modules: transmitters, receivers and transceivers. In transmitters, electrical signals carrying information enter via the interconnects 108, are amplified using the laser driver circuitry and are transmitted in the form of light signals using the laser chips. In receivers, optical signals carrying information are received by the photodiodes and amplified by the electronic circuitry to be delivered to the host board via interconnects 108. A fiber-optic transceiver performs the transmitting and receiver function in the same module (housing). The performance requirements on transceivers are more stringent than on transmitter and receiver modules because of the electrical crosstalk between the transmitter and the receiver chips and traces. Namely, in small dimensional environments, such as, fiber-optic modules, the large signal amplitudes required to drive lasers and the electrical transmission lines on the FPCB easily couple to the sensitive inputs of receivers, if they are physically close. Electromagnetic crosstalk is a difficult design problem in fiber-optic modules. Channel to channel crosstalk and transmitter to receiver crosstalk within the module can severely reduce the performance of the fiber-optic component.
The trace density traversing a bend in a conventional FPCB 100 is limited by the width of the traces, the thickness of the flexible circuit board and the number of layers. The mechanical and electrical characteristics of the traces and the FPCB have a strong influence on the high-speed electrical properties of the interconnect to the host board. A number of approaches have been explored in an attempt to maximize the density of traces in high-speed applications. In very high-speed systems, the transmission line impedance must be controlled (and constant) to prevent signal reflections that degrade the signal shape, while the information is typically carried with two signals of opposite polarity on two traces, also known as differential-signal traces. Typically, the impedance of high-speed signal traces is controlled by using at least one electrically conductive layer adjacent to the layer with the signal traces. There are additional constraints on differential-signal traces: The signal propagation delay on the two traces must be equal to prevent the introduction of jitter. Furthermore, in most applications, the traces carrying differential signals cannot be electrically coupled to each other, in which case additional ground traces have to be introduced between the two signal traces. All of these requirements result in designs with tightly controlled transmission-line impedances and propagation constants with added traces and layers for grounding. In order to reduce the size of such systems, the designer has few options: a reduction in trace width increases the trace resistance and inductance which alters the characteristic impedance of the lines that in turn can be corrected by decreasing the thickness of the insulating layer of the flexible circuit or increasing the dielectric permittivity of the flexible circuit material, or by employing a combination of both. Decreasing the trace width increases the trace losses, which in turn degrades the signal integrity. Adding high-speed signal layers doubly increases the thickness of the FPCB because each new high-speed signal layer requires a ground plane. This in turn increases the rigidity of the flexible printed board and the minimum bend ratio. FPCBs with more layers are hence more difficult to bend and fit into many of the present-day fiber-optic module form-factors. Adding more layers also increases the need for using vias. Maintaining the transmission line impedance and the propagation constant through a via is very difficult and not easily corrected. Increasing the density of the traces, furthermore, increases the possibility of trace-to-trace electrical crosstalk. Therefore, the conditions on performance and the mechanical rigidity of the FPCB collectively place a limit on the number of traces that can traverse a bend.
Accordingly, there exists a need for an improved fiber optic module design utilizing a flexible printed circuit board. The improved FPCB should provide interconnects between the optoelectronic and electronic chips and the module interconnects without the use of multiple trace layers and should also provide total trace counts that exceed the conventional single-layer flexible printed-circuit board fiber-optic module design while still delivering the required trace transmission performance necessary for high-speed performance of the fiber-optic module. The present invention addresses such a need.